In recent years, in order to perform power supply or perform charging, wireless power transfer techniques have been gaining attention. Research and development are being conducted regarding a wireless power transfer system wirelessly performing power transfer to various electronic apparatuses such as mobile terminals and notebook computers and household electrical appliances or to power infrastructure equipment.
When wireless power transfer is used, standardization is preferably performed so that power sources which transmit power and power receivers which receive the power transmitted from the power sources are used without trouble even when they are products manufactured by different manufacturers.
Generally, as wireless power transfer (wireless power transmission or wireless power supply), techniques which use electromagnetic induction or radio waves are known, and wireless power supply is getting prevalent in electronic devices such as portable terminals.
Recently, wireless power transfer techniques using strong coupling resonance have been attracting attention as techniques being capable of transferring power to a plurality of power receivers while placing each power receiver at a certain distance from a power source, or to various three-dimensional positions of each power receiver. Wireless power transfer techniques using magnetic field resonance or electric field resonance, for example, are known as this kind of wireless power transfer using strong coupling resonance.
In other words, as schemes for transferring power on the order of several watts at a distance of several centimeters to several tens of centimeters, magnetic field coupling schemes which apply coils to both power sources and power receivers, for example, are commonly used. Such power transfer schemes which use magnetic fields include the conventionally known electromagnetic induction scheme, and a magnetic field resonance scheme recently proposed by MIT (Massachusetts Institute of Technology) in the U.S.A.
The electromagnetic induction scheme includes, for example, the Qi (Chi) (registered trademark) standard introduced by the WPC (Wireless Power Consortium), and the magnetic field resonance scheme includes, for example, the WiPower (registered trademark) standard introduced by the A4WP (Alliance for Wireless Power).
In these wireless power transfer techniques, standardization is under development assuming as targets extra 100-W household electrical appliances, including microelectronic devices on the order of several watts in the phase of practical application. The development of wireless power supply techniques on the order of several kilowatts is also making progress for electrical vehicles mainly by automobile manufacturers.
Conventionally, in order to perform power supply or perform charging, various wireless power transfer techniques for wirelessly transferring power have been proposed, as described earlier. Among these techniques, the magnetic field resonance scheme advantageously allows the power supply distance to be longer than the electromagnetic induction scheme and the degree of freedom to be high in terms of the positions and the postures of power receivers.
The magnetic field resonance scheme may also implement power supply from one power source to a plurality of power receivers, and therefore promises to expand the market with an improvement in convenience in the future. Especially for the postures of power receivers, a three-dimensional wireless power transfer technique has also been proposed for generating an appropriate synthetic magnetic field by control according to the postures of the power receivers while synchronizing outputs from power source coils to perform highly efficient power supply to the power receivers.
In the magnetic field resonance scheme, since a near-field magnetic field is used to transmit energy, a problem in principle is posed in which the power transfer efficiency varies depending on the posture and the distance between the power source and the power receiver. It is, therefore, difficult to set precise power supply conditions unless the posture and the position of the power receiver relative to the power source are detected in any way.
In power transfer which employs the magnetic field resonance scheme, a method of conducting test power transfer using weak energy at the start of power transfer to achieve appropriate output setting of the power source in a cut-and-try manner is conceivable.
However, when, for example, three-dimensional wireless power transfer is performed using a synthetic magnetic field generated by power sources, the control method is so complex that a method of calculating the postures and the positions of the power receivers relative to the power sources, especially, their initial values is demanded.
In other words, when power transfer (power supply) is started with the posture and the position of the power receiver relative to the power source being left unknown, it takes a long time to perform processing such as test power transfer, resulting in considerable delay in actual power transfer from the power source to the power receiver. Although wireless power transfer which uses magnetic field resonance will be mainly taken as an example in this specification, the application of the present embodiment is not limited to magnetic field resonance, as a matter of course.
In the related art, various wireless power transfer techniques are proposed.
Patent Document 1: Japanese Laid-open Patent Publication No. 2012-023950
Non-Patent Document 1: A. Kurs, et al., “Wireless Power Transfer via Strongly Coupled Magnetic Resonances,” SCIENCE Vol. 317, pp. 83-86, Jul. 6, 2007
Non-Patent Document 2: “System Description Wireless Power Transfer,” Wireless Power Consortium, Volume I: Low Power, Part 1: Interface Definition, Version 1.1.2, June 2013
Non-Patent Document 3: R. Tseng, et al., “Introduction to the Alliance for Wireless Power Loosely-Coupled Wireless Power Transfer System Specification Version 1.0,” (Reprinted from) IEEE Wireless Power Transfer Conference 2013, Technologies, Systems and Applications, pp. 1-6, May 15-16, 2013
Non-Patent Document 4: J. Nadakuduti, et al., “Operating Frequency Selection for Loosely Coupled Wireless Power Transfer Systems with Respect to RF Emissions and RF Exposure Requirements,” (Reprinted from) IEEE Wireless Power Transfer Conference 2013, Technologies, Systems and Applications, pp. 1-6, May 15-16, 2013
Non-Patent Document 5: “A4WP Wireless Power Transfer System Baseline System Specification (BSS),” TWC of A4WP, Ver. 1.2, Nov. 21, 2013